1. Field of the Invention
This invention relates to ring laser gyroscope output optics detection systems, and more particularly, it relates to an output optics system for a multioscillator sensor which provides a local oscillator-based electronic separation of Sagnac effect modulated Faraday biased beat frequency signals without the need for complex optical signal processing and components.
2. Description of the Related Art
The Ring Laser Gyroscope has been developed as a logical replacement for the mechanical inertial gyroscope. Based upon the principles of the Sagnac Effect, ideally the ring laser gyroscope has minimal moving parts allowing extremely accurate rotational sensing. As originally envisioned, the ring laser gyroscope has at least two counter-propagating electromagnetic waves (such as light) which oscillate within an optical ring cavity. When the ideal ring laser gyroscope is stationary, no rotation is indicated by the sensor. As the ring cavity of the laser gyroscope is rotated about its central axis, the counter-propagating waves interact so that a beat frequency is developed. A linear relationship between the beat frequency and the rotation rate of the gyroscope with respect to the inertial frame of reference may be established.
Although the ideal ring laser gyroscope is characterized by a beat note proportional to the rotational rate, the two mode planar ring laser gyroscope requires rate biasing or mechanical dithering to prevent counter propagating waves from locking at low rotation rates. Mode locking is a major difficulty at low rotation rates where the ring laser gyroscope produces a false indication that the device is not rotating. If the rotation rate of a ring laser gyroscope starts at a value above that of where lock-in occurs, and is then decreased, the frequency difference between the beams disappears at a certain input rotation. This input rotation rate is called the lock-in threshold. The range of rotation rates over which lock-in occurs is generally called the dead band of the ring laser gyroscope. Lock-in arises from the coupling of light between the beams. Today, the only means of overcoming the lock-in effect of the counter-propagating modes of light within a two mode gyroscope is to mechanically dither the mirrors or body of the gyroscope or to apply a continuous angular rate to the gyroscope. A more detailed explanation of the problems associated with a planar two mode gyroscope are described in Laser Applications, edited by Monte Ross, pages 133-200 (Academic Press, 1971).
Since one of the primary benefits of a ring laser gyroscope is that it overcomes the need for mechanical or moving parts, a body dithered planar two mode gyroscope does not truly meet this goal. In an effort to achieve a fully optical ring laser gyroscope, the out-of-plane multi-mode or multioscillator ring laser gyroscope was developed to overcome the effects of mode locking without the need to dither. The terms "multimode" and "multioscillator" refer to four modes of electromagnetic energy that propagate simultaneously in the cavity as opposed to the usual pair counter-propagating linearly polarized modes that exist in the conventional two mode gyroscope. A detailed discussion of the operation of the multi-oscillator laser gyroscope is presented in the article by Chow, et. al., at pages 918-936, IEEE Journal of Quantum Electronics, Vol. QE-16, No. 9, September 1980. In an effort to solve the lock-in problem, non-planar multioscillator ring laser gyroscopes have been developed, having more than one pair of counter propagating modes.
Briefly, the basic multi-oscillator ring laser gyroscope operates with left circularly polarized (LCP) and right circularly polarized (RCP) light beams and uses a Faraday effect glass device within the cavity or magnetic field on the gain plasma to provide a frequency shift between the counter-propagating waves to prevent mode locking. An example of this theory of multioscillator ring laser gyroscope may be found in U.S. Pat. No. 4,818,087 entitled "ORTHOHEDRAL RING LASER GYRO" issued Apr. 4, 1989 to Raytheon Corporation (Terry A. Dorschner, inventor); and U.S. Pat. No. 4,813,774 entitled "SKEWED RHOMBUS RING LASER GYRO" issued Mar. 21, 1989 to Raytheon Corporation(Terry A. Dorschner, et. al., inventor). The nonplanar ray path produced in a multioscillator ring laser gyroscope ensures circular polarized reciprocally split light. The nonplanar ray path reciprocally rotates the polarizations by many degrees yielding the necessary high purity circular polarization. The nonplanar reciprocal phase shift also achieves two Faraday biased gyroscopes, the gain curve 10 of which is illustrated in PRIOR ART FIG. 1. The nonplanar ray path splits the light through its geometry into two separate gyroscopes, one being left circulatory polarized and the other right circulatory polarized. This splitting is known as reciprocal splitting and typically is in the range of 100 MHz-1,000 MHz. By placing a Faraday element in the beam path of a nonplanar ring laser gyroscope, when the proper magnetic field is applied to the Faraday glass element, nonreciprocal splitting of each gyroscope is achieved. As shown in FIG. 1, at least four modes are produced: a left circularly polarized anti-clockwise (L.sub.a) frequency 12, a left circularly polarized clockwise (L.sub.c) frequency 14, a right circularly polarized clockwise (R.sub.c) frequency 16, and a right circularly polarized anti-clockwise (R.sub.a) frequency 18. The Faraday splitting (between 12 and 14, shown as GYRO 1, and 16 and 18, shown as GYRO 2) between clockwise and anti-clockwise modes is about 1 MHz. At least four corner mirrors, 22, 24, 26, and 28 form the ring resonator path, which contains the two gyroscopes symbolized by their respective gain curves of FIG. 1. The partially transmissive corner mirror 28 allows light to leave the resonator and fall upon the prism 30 and onto a set of photo-electronic transducers, such as the quadrature-spaced photodetectors 44 (HET A and HET B) for signal processing. (These photodetectors are usually, but not limited to, photodiodes.) When the signals are subtracted during the electronic processing to remove the Faraday bias, the scale factor of the gyroscope is doubled over the conventional ring laser gyroscope. The nonplanar geometry multioscillator ring laser gyroscope using a Faraday element is currently designed using a gas discharge pump to provide the active medium, which medium occupies a portion of the light beam path. Reflections and backscatter from the intra-cavity element and instabilities of the magnetic field associated therewith cause difficulties that need to be overcome in order to build a fully optical navigational grade multi-oscillator ring laser gyroscope.
In the planar ring laser gyroscope or the multi-oscillator laser gyroscope system, it is necessary to extract a portion of each beam propagating within the laser cavity to produce output signals, which represent the difference in frequency between wave pairs having the same sense of polarization within the cavity (one pair for the planar configuration; two pairs for the multi-oscillator). For example, in planar ring laser gyroscope systems, rotational information is obtained by combining the oppositely directed waves. In the ideal case of a uniformly rotating laser, the frequencies of the waves are slightly different.
The planar gyroscope has a device for combining its oppositely directed beams to obtain a read out which includes a dielectric mirror mounted on one side to the ring laser gyroscope body. Mounted to the opposite surface of the mirror, a prism assembly (which preferably may be an upright symmetric prism) is used to form a fringe pattern. The prism is directly mounted to the mirror to minimize vibrations.
In the planar gyroscope output optics, the fringes are a measure of the instantaneous phase difference between the oppositely directed beams. For the case when the intensities are matched and counter propagating beams are nearly collinear, the fringe pattern is stationary. When the laser gyroscope is rotated, the fringe pattern moves at the beat frequency rate. If the fringe spacing is considerably larger than the dimensions of a photodetector, a measurement of the rotation rate can be made by simply recording the rate at which the intensity maximum moves past the detectors.
The direction is which the fringe pattern moves past the detectors determines the sense of rotation. By using two detectors spaced at 90.degree.; or a quarter fringe apart, and a logic circuit, both positive and negative counts can be accumulated to give rotation rate and sense. It should be noted that with this type of readout, the laser gyroscope is inherently a rate integrating gyroscope with a digital output. Thus, with up-down counting, the net number of accumulated counts depends only on the net angle through which the ideal gyroscope is rotated. One complete revolution of the gyroscope would produce on the order of 10.sup.6 counts. In summary, the output optics detection system for the planar ring laser gyroscope is relatively straight forward.
The same cannot be said for the multioscillator ring laser gyroscope. Multimode ring laser gyroscopes as known in the art may employ optical crystals and Faraday effect devices to shift the frequency of the laser beams. Heretofore, the biasing and detection schemes which have been proposed have been unduly complex and have had high noise levels associated with them. This was acknowledged as early as 1977 in U.S. Pat. No. 4,123,162 issued to Sanders and assigned to the common assignee of this application. In order to solve the problem of biasing and detecting output signals from a multioscillator ring laser gyroscope, the Sanders '162 patent was directed to a scheme of rotation direction determination through a circuit which dithered the laser plasma current, and used the AC component from the plasma power supply as a phase standard for detecting the sign or direction of rotation of the ring laser gyroscope. Sanders '162 superimposes a differential AC dithering voltage onto the DC voltage of the plasma power supply. A phase reference voltage is synchronized with the AC dither of the plasma and is applied to the synchronous demodulator 78 of Sanders '162. A slight change in the plasma current reduces one beat frequency (characteristic of one gyroscope contained within the multioscillator) and increases another beat frequency. The Sanders circuitry determines the direction of rotation by determining whether the signal is in phase or out of phase with the phase reference signal. Sanders '162 uses a single photodetector to achieve its rotation rate and rotation sense measurements. Sanders '162 also discloses a maximum intensity seeking path length control servo which is not easily adaptable by most multioscillators used due to the complex nature of the intensity curves exhibited in such a scheme.
Another scheme for rotation rate and rotational direction sensing is disclosed in the following U.S. Pat. Nos. 4,415,266; 4,429,997; and, 4,449,824, all issued to Matthews. Patent '266 and Patent '997 are directed to a phase-locked loop system for a multioscillator ring laser gyroscope, while the U.S. Pat. No. '824 is directed to the structure of the output optics. A complex output optics detector prism structure is disclosed by Matthews, which includes three mirrors (22, 40, and 41), a beamsplitter (42), a set of quarter-wave plates (43 and 53), a set of polarizers (44 and 54), and a set of detector diodes (45 and 55) (as shown in FIG. 2 of the U.S. Pat. No. 4,449,824). The electronic signal processing systems disclosed in the '266 and '997 patents are used to process the heterodyned optical output signals provided by complex optics as discussed in the '824 patent. The Matthews' patents are all directed to an overall system which requires complex optics to separate the beam pairs in the multioscillator. Matthews employs a path length control system which compares the optical intensity of each pair of signals to produce a path length control error signal. The problems which arise when using complex output optics (besides the difficulty of manufacturing a bulky mechanical structure and optical alignment) include severe optical signal attenuation, and measurement accuracy problems associated with optical signal backscatter. It therefore is desirable to provide an output optics structure and system which is free from the confinement of complex optical signal processing.
One attempt to simplify the output optics system is disclosed is U.S. Pat. No. 4,836,675, issued Jun. 6, 1989 (Martin and Hendow, inventors) and assigned to the common assignee of this application. In this case, the applicants used straight forward optics (similar to the output optics used in a dithered planar ring laser gyroscope system) and rather complex electronics to achieve the goal of measuring rotation rate and sense, as well as achieving cavity length control, in a multioscillator ring laser gyroscope system. The system that U.S. Pat. No. 4,836,675 discloses for cavity length control attempts to discriminate the amount of envelope modulation depth to determine the gyroscope's operating point, using no additional photodetectors than what is required for a planar gyroscope; however, the proposed electronic system for processing the optical output signals are rather complex, and therefore subject to signal degradation and noise, as well as higher cost implementation.
U.S. patent application Ser. No. 07/464,164, filed Jan. 12, 1990, now U.S. Pat. No. 5,116,132 granted May 26, 1992, discloses an alternative system of Four mode detection which uses RC circuits to separate heterodyne signals received from the heterodyne optical detectors. The method disclosed in the referenced patent must account for significant cross-talk phenomena arising when the heterodyne detectors are not exactly 90.degree. apart. Also, the system disclosed in the referenced patent requires precision adjustments under severe bandwidth constraints.